The present invention relates to a method of inspecting masks used in manufacturing LSI or the like, and also to an apparatus for performing the method.
Conventionally, to inspect the pattern formed on a photomask or the like, light having a wavelength of 0.4-0.7 .mu.m is applied to one major surface of the mask, and the amount of light passing through the mask is detected. Since the amount of light changes according to the shape of the pattern, the presence or absence of the pattern and the dimensional accuracy of the pattern can be determined from the detected amount of light.
In practice, the mask is positioned in a plane which is perpendicular to the light beam, and is moved parallel to the plane while the light is being applied to it. The amount of the light passing through the mask is detected, and a first signal (i.e., detection signal) is generated from the detected amount of light. The first signal is compared with a second signal (i.e., reference signal) obtained from the design data used in forming the pattern. This conventional method is disclosed in Daikichi Awamura, Reticle Inspection Technology to Compare the Pattern against Data, Proc of SPIE, Vol., 334, 1982, p. 208.
With the method it is impossible to detect the pattern defects of less than about 1 .mu.m, due to the interference or diffraction of light. This limited accuracy of inspection is sufficient in the photo-optical reduction exposure system, generally called stepper, which reduces and projects a mask image about one-fifth to one-tenth the original mask size onto a wafer.
The recent trend is that ultra LSI having wires less than 1 .mu.m broad are developed. It is now expected that X rays, electron beams or ion beams are used to transfer a pattern to a wafer in the process of manufacturing ultra LSI. The conventional reduction transfer of the pattern image can no longer be applied, and the unit magnification pattern must be transferred to the wafer. Hence, the pattern defects must be detected and corrected to the accuracy of 0.1-0.2 .mu.m. The conventional method of inspecting masks and patterns, which uses a light beam, can not obtain such high-accuracy detection of pattern defects.